A neutron diffraction study of Fe7Se8

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Abstract
An investigation of Fe7Se8 was undertaken to study the magnetic ordering and phase transitions in this compound. On a sample slowly cooled from 600 °C a superstructure was identified corresponding to the 3c structure found by Okazaki [4]. This is derived from the small Ni-As-type cell by doubling of the a-axis and tripling of the c-axis. Neutron diffraction diagrams were made at several temperatures between 4.2 °K and 493 °K. The unit cell dimensions found were : The space groups C 46 — P62 and C 56 — P64 proposed by Okazaki and Hirakawa [3] are not applicable. A possible space group is P31, with vacancies at (1/2, 1/2, 0) (1/2, 0, 1/3) (0, 1/2, 2/3). The magnetic moments are ferromagnetically aligned within each (0001) sheet and antiferromagnetically aligned in neighbouring sheets. At low temperatures the moment direction is perpendicular to the c-axis, but between 120 °K and 130 °K the moments turn to a direction closer to the c-axis. The intensity of the (0003) reflection indicates a tilt of the spin with respect to this axis. No indication has been found of the gradual change in the axis of easy magnetization which was measured by Hirakawa [5] on a torque magnetometer

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